Multicomponent force transducer

ABSTRACT

A piezotransducer device in which several piezoplates are located between force-transmitting members and are oriented with the force-sensitive axes depending on the type of force to be measured thereby.

United States Patent Inventors Hans Conrad Sonderegger Neitenbach; GelliSpescha, Winterthur, both of Switzerland Appl. No. 846,018

Filed July 30, 1969 Patented Oct. 19, 1971 Assignee Ristler InstrumentsA. G.

Wintertllur, Switzerland Priority July 30, 1968 Switzerland 1 1446/68MULTICOMPONENT FORCE TRANSDUCER 13 Claims, 12 Drawing Figs.

US. Cl 3l0/8.6, 310/8.7, 310/96 Int. Cl HOlv 7/00 Field of Search 3I0/8.2,

[56] References Cited UNITED STATES PATENTS 3,151,258 9/1964 Sondereggeret a1. 310/8.7 3,320,582 5/1967 Sykes 340/10 2,368,609 1/1945 Burkhardt310/86 3,104,334 9/1963 Bradley et a1.... 310/8.4 3,358,257 l/l967Painter et al..... 338/5 2,875,352 2/1959 Orlacchio.... 310/8.12,774,892 12/1956 Camp 310/81 3,183,378 5/1965 McCracken et al. 310/8.7OTHER REFERENCES Eldon Eller, Squeeze Electricity, International Scienceand Technology, July 1965, pp. 32- 38.

P. J. Ottowitz, A Guide to Crystal Selection," Electronic Design, May10, 1966, pp. 48- 51. 310/96 Primary Examiner-Milton O. HirshfieldAssistant Examiner-B. A. Reynolds Attorney-Craig, Antonelli & HillABSTRACT: A piezotransducer device in which several piezoplates arelocated between force-transmitting members and are oriented with theforce-sensitive axes depending on the type of force to be measuredthereby.

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I INVENTOR-S HANS (Lo/(R40 smvosnsoqsn 4-1-4 Gnu ifg-"L'LHA 7 lMULTICOMPONENT FORCE TRANSDUCER The present invention relates topiezotransducer units, and more particularly to a piezotransducer unitcomprising a piezoelement mounted between two members adapted totransmit a forceapplied thereto to the piezoelement.

, In the field of measurements, problems must often be solved in whichmore than one component of a force must be measured. Such forces may becompression, tension or shearing forces as well as moments. Knownmeasuring arrangements for such multiple component measuring systems aregenerally constructed in accordance with the strain gauge strip method.Accordingly, the measuring device receiving the various forces must bemechanically worked in such a manner that individual components can bedetermined separately, thus necessitating a very complicatedconfiguration. The reason for this is in particular that the variouscomponents must be prevented from affecting each other. Owing to thenecessity that the measuring device must be divided mechanically intovarious stress components unintentionally a constructional form resultswhich is difficult to produce and in particular a resilient structurewhich generally has totally different degrees 'of resilience in thevarious directions of the components and thereby a low and irregularnatural frequency character. Moreover, the production of such multiplecomponent measuring devices based on strain gauge strips is veryexpensive and not universally applicable. Admittedly, further forcemeasuring methods are in existence, e.g. operating on an inductive orcapacitative basis; these, however, are substantially never used formulticomponent force measurement.

The piezomeasuring technique provides better measuring conditions. Owingto the fact that piezocrystals can be used which are produced in variouscut directions and which are suitable for the measurement of compressionforces as well as shearing forces, simple stable constructions areobtained. Owing to the fact that in the piezomeasuring technique forcescan be measured directly without the intervention of an elongation orother stress, this system is particularly well suited for themeasurement of forces because piezocrystal cross sections can be used,whereby simultaneously a very high sensitivity, yet very great rigiditycan be obtained. The ratio of rigidity to sensitivity obtainable therebycannot be attained even approximately with any other system.

According to the invention, piezotransducer units are proposedcomprising substantially two force-transmitting plates between which aplurality of piezoelectric plates are located which are mutuallyinterchangeable and which can be assembled to form a transducer unitresponsive to compression, shear or torque, dependent upon theorientation of the force sensitive axes. According to the invention,thus three basic elements are available in a simple manner, which differfrom each other only by the direction of sensitivity of the respectivecrystal plates and which can be combined by mechanical series connectionin such manner that any desirable multicomponent measuring valueconverter can be produced.

Several embodiments of the invention will be described below by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 illustrates a cross section through a perforated disclikepiezotransducer unit,

FIG. 2 is a section along line AA in FIG. 1,

FIG. 3 illustrates a cross section of a different constructionalarrangement of a disclike piezotransducer unit,

FIG. 4 is a section through the same piezotransducer unit along line 8-8in FIG. 3,

FIG. 5 illustrates a piezocrystal from which two piezodiscs have beenproduced in different cut directions,

FIG. 6 illustrates a section through a piezodisc suitable formeasurement of a compression force,

FIG. 7 illustrates a section through a piezodiac suitable formeasurement of a shearing force,

FIG.'8 illustrates an embodiment suitable for the measurements of twoforce components, such as compression Z and moment M,

FIG. 9 illustrates an embodiment suitable for the measurement of twoforce components, such as shearing force X, and compression force Z,

FIG. 10 illustrates an embodiment suitable for the measurement of threeforce components and a moment,

FIG. 11 is a cross section through an embodiment of a piezotransducerunit suitable for the measurement of a moment, and

FIG. 12 is a cross section through an embodiment of a piezotransducerunit suitable for the measurement of a shearing force.

FIG. 1 illustrates a cross section through an embodiment of apiezomeasuring cell or transducer unit which is adapted for themeasurement of rotary moments. The unit consists of annularforce-transmitting plates 1 and 2 which are connected to each other by athin tubular inner jacket 3 and a tubular outer jacket 4, exerting amechanical bias or stress on the plates. The connection is effectedpreferably by annular welds 5, 6 and so on. Piezodiscs or crystals 7 aredisposed in direct contact with the transmitting plate 1 by whichpositive charges produced under the effect of a rotary moment aretransmitted directly to a casing 24 shown in FIG. 2 and thence to athreaded potion 8 of a connecting terminal. A ringlike electrode 9 islocated on the other side of the crystals 7 and receives thecorresponding negative charges which are transmitted to a centralcontact member 10 in the connecting terminal. A disclike plate 11consisting of a highly insulating material, e.g. aluminum oxide, liesbetween the electrode 9 and the force-transmitting plate 2. However,other highly insulating and extremely rigid insulating materials arealso known and may be used instead of aluminum oxide. The piezocrystals,the electrode and the insulating disc are centered by an innerinsulating ring 12 and an outer insulating ring 13 in such a manner thatcontact with the walls at the inner and outer peripheries is avoided.

The whole crystal unit can be easily conveyed from one assembly stationto another during the manufacturing process owing to the presence ofthese centering rings.

FIG. 2 illustrates the piezotransducer unit in section along the lineA-A in FIG. 1. The outer tubelike wall 4 which is as thin as possible aswell as an inner wall 3, which as also as thin as possible, and theouter and inner isolation and centering rings 13 and 12 are shown insection. The push or shear sensitive axes of the piezodiscs 7 areindicated by respective arrows.

These axes are placed during the assembly in exactly tangentialdirections to a circle of mean diameter D. In this manner, eachpiezodisc 7 is subjected to and stressed by a shearing force when arotary moment is applied to the forcetransmitting plates 1 and 2 wherebythe discs deliver a corresponding charge to the electrode 9 and thus tothe central contact member 10 of the connecting tenninal.

FIG. 3 illustrates a simpler construction of a piezotransducer unit ormeasuring element in which the space between two force-transmittingplates 31 and 32 and the inner parts is filled with an epoxy resin 33.The transmitting plates 31 and 32 consist of an insulating material,e.g. aluminum oxide. A ring plate 34 of metal has a lug 35 to which ametal screen 36 of a connecting cable 37 is attached such as bysoldering. An inner conductor 38 of the cable is connected to a disclikeelectrode 39. Piezodiscs 40 are located between the ring plate 34 andtheelectrode 39.

FIG. 4 illustrates the piezomeasuring element in section along the line8-8 in FIG. 3. The push or shear sensitive axes of the individualpiezocrystal disc 40 are indicated again by arrows. During assembly themeasuring elements are deposited in such a manner that all the axes areplaced exactly parallel to an X-axis whereupon the disclike electrode 39and the transmitting plate 31 and deposited on the assembled discs 40.Thereafter, the whole measuring element is subjected to a vacuum' whileit is located in a special mold, and is impregnated with a highlyinsulating epoxy resin. The connector portion 49 is thereafter embeddedin a silicone rubber 50. In this manner, a piezomeasuring element can beproduced with simple means which is sensitive to push or shear along theaxis X. Owing to physical properties of the piezocrystals forces alongthe Z and Y axes have no signal-producing effect.

It is obvious that the two constructional forms described with referenceto FIGS. 1, 2 and FIGS. 3, 4 can be assembled at choice for themeasurement of moments or shearing forces. Also a second series ofsimilarly directed piezodiscs, such as discs 7, can be substitutedwithout difficulty for the insulating ring 11. In this manner, twice thesignal can be taken off electrode 9 between the two series of crystaldiscs. This arrangement is used very much and avoids the need forinsulating discs. The two piezotransducer units according to theinvention differ substantially only by the orientation of the axes ofthe individual crystal discs, as may be seen clearly also from FIGS. 2and 4.

FIG. 5 illustrates, by way of example, a natural quartz crystal 51 inwhich the known axes X, Y and Z are shown. For producing a disc which issensitive to pressure P, such as shown; in FIG. 6 a piezodisc 52 must becut from the crystal in the plane Y, Z. The force must be appliedparallel to the axis a X and the electrical charges are produced on theupper and lower disc surfaces.

In order to produce a piezodisc which is responsive to a push orshearing force, such as shown in FIG. 7 a disc 53 must be cut from thecrystal 51 in the plane X, Z. The disc 53 (FIG. 5 is then sensitive topush or shearing forces P in the direction of the axis X, as shown inFIG. 7. COrresponding charges are delivered at the circular upper andlower limiting surfaces.

The discs 52 and 53 are insensitive for forces in the Y and Zdirections. Similar force orientations can be obtained also with othercrystals, and it is also possible to obtain such effects also withpiezoceramic discs. Furthermore, semiconductor crystals with similarsensitivity can be produced which have piezoresistive properties.

FIG. 8 illustrates the use of two piezomeasuring elements in anarrangement wherein machining experiments are to be carried out on atest piece 81 be means of a drilling or milling tool 82. In this case,the rotary moment M and the feed force in the Z direction are to bemeasured. For M pf an arrangement a piezotransducer unit 83, includingtransmitting plates 83a and 83b for measuring torque according to FIGS.1 and 2 is assembled together with a compression force-measuringtransducer unit 84 of generally similar construction and shown in blockform, the two transducer units being clamped between the test piece 81and a support 86 by means of a stressing screw 85. An output signalcaused by the prestressing can be reduced to zero by any known means. Incontrast the feed force of the drill in the Z direction, as well as thereaction moment M of the drill 82 can be registered completelyindependently of each other with extremely high resolution. Owing to thelarge cross section of the crystals, the whole measuring system becomesextremely rigid, whereby also force fluctuations and moment variationsof very high frequency can be measured.

FIG. 9 illustrates a further example for use in an arrange ment whereinmilling or grinding tests are to be carried out on a testpiece 91, andwherein forces in the Z and Y directions are to be measured. For thispurpose a piezotransducer unit 94 for measuring pressure and apiezotransducer unit 93 for measuring a shearing force according toFIGS. 3 and 4 is used in a similar manner, and the units are clampedagain between the testpiece 91 and a support 96 by means of a clampingor stressing screw. The force-transmitting plates 93a and 93b of unit 93are shown for clarity.

FIG. 10 illustrates, by way of example, a test arrangement whereinparticles 102 impinging upon a testpiece 101 produce correspondingreaction forces; the components of these forces are to be measured inall three directions X, Y and Z, and additionally also the moment M isto be determined. By mechanically connecting in series individualpiezomeasuring elements 104, whose transmitting plates 104a and lMb areshown for measuring force along the Z axis, 105 for measuring shearingforce along the Y axis, E06 for measuring shearing force along the Xaxis and 107 for measuring moment, the

complicated measuring problem becomes a very simple matter. In this casethe whole measuring arrangement becomes a rigid high frequency deviceowing to the prestress bias provided by a screw 108.

FIG. 11 illustrates in cross section a further embodiment of apiezomeasuring cell or transducer unit for the measurement of rotarymoments or torque. A base plate provided with two tubelike wall members112 and 113 has a U-shaped cross section. A force-transmitting plate 114is connected to two thin tubular ring members 115 and 116 which in turnare connected by rings welds 117 and 118 to the wall members 112 and 113constituting the limbs of the U-shaped cross section of the baseplate110. This fold construction is effected in such manner that thetransmitting plate 114 is pressed with bias against a measuringarrangement and the base plate 110. The measuring arrangement comprisesan electrode plate 120, an insulating plate I21, and a plate 119 withpiezodiscs which may be embedded in a layer of, e.g. an epoxy resin.

The electrode plate 120 is connected to a connector 123 by means of aplug contact 122. In place of the insulating plate 121, alternatively aplate with piezodiscs may be used. In this case the plate may beprovided with piezodiscs the sensitivity axes of which have differentdirections form the axes of the piezodiscs of the plate 119. The crystalarrangement is centered and detained by two insulating rings H24 and125. However, the crystal discs may be cast in an epoxy resin disc asillustrated in FIGS. 3 and 4. Owing to the resilient connection betweenthe transmitting plate 114 and the baseplate 110, the sensitivity of themeasuring arrangement for rotary moment is altered only very little,because the stiffness thereof is considerably higher.

A further possible embodiment of a peizomeasuring cell which can be usedfor measuring rotary moments as well as shearing forces and pressureforces is illustrated in FIG. 12. In contrast to FIG. ll a transmittingplate 131 is connected for resilient yield along two axes X and Y to aU-shaped base plate 130. In this case, the ends of the thin wallsconstituting the limbs of the U-shaped cross section of the base plateare provided with annular enlargements 132 and 133 to which thin walledrings 135 and 136 attached to the transmitting plate 131 are weldedunder stress in the X direction. Thereby, annular gaps I34 and 137 areproduced which afford resilience to the transmitting plate 131 in the Ydirection. The measuring arrangement is substantially the same asdescribed with reference to FIG. 11.

Simple embodiments, however, may be obtained also when the individualpiezotransducer units or measuring elements are constructed withoutcentral opening. The problem of prestressing or bias must then be solvedby an externally applied force, e.g. a sleeve. Obviously, suchembodiments and applications fall also within the scope of theinvention.

Even further combinations can be devised for use of the individualmeasuring cells and are also part of the invention. Thus two or moreindividual cells or units may be disposed in a common housing andseparate connections to the individual component arrangements may beprovided. In the first place, quartz is a suitable material for apiezocrystal for the intended use. The invention, however, can beperformed without difficulties with any other piezoelectrical materials.In place of individual piezocrystals, piezoresistive crystal discs, akinto semiconductors may also be mounted in the elements or units accordingto the invention. However, multiple core connection conditions arisethen.

It will be clear that the invention permits any multiple force componentmeasurements and moment measurements in test articles to be effected ina simple manner in that a rigid measuring structure can be obtained by acombination of individual piezomeasuring elements and by mechanicallystressing them. The construction of the individual piezomeasuringelements or transducer units may be effected in accordance with uniformprinciples depending upon whether the element is to be used for themeasurement of pressure forces, shearing forces or moments, in that thedirections of the sensitivity axes of the piezocrystals are suitablyaligned during assembly. Furthermore, individual crystals with adifferent shape, e.g. rectangular or trapezoidal shape, can be used inplace of the circular disclike crystals. From the point of view ofproduction, however, a circular disc in considerably simpler andcheaper. Also the manner of construction of the piezomeasuring element,whether it is constructed according to the principle in accordance withFIGS. 1, 3, II or FIG. 12, has no effect on the idea of the invention.It is also within the scope of the invention that only one of the twopiezomeasuring cells illustrated in any of FIGS. 8 to [0 and havingcrystal arrangements according to FIG. 2 or FIG. 4 may be assembled inone measuring device. The piezocells can thus be utilized individuallyor in any combination in accordance with the requirements of themeasurement to be effected.

We claim 1. A piezomeasuring device with a force-receiving body formeasuring a plurality of forces impinging thereon having a plurality ofseparate transducer units mounted together, each transducer unitcomprising:

a pair of force-transmitting members disposed about a common axis; and aplurality of piezocrystals disposed between the forcetransmittingmembers of said pair and being sensitive to forces in a single directionwith respect to said axis;

wherein the crystals of each separate transducer unit are sensitive toforces in a direction different from the direction of sensitivity of thecrystals of the other transducer units, whereby said measuring deviceprovides a compact arrangement sensitive to forces in a plurality ofdirections.

2. A piewmeasuring device according to claim 1, wherein said pair offorce-transmitting members of each transducer unit are annular plates,between which said crystals are arranged, said crystals beingdisc-shaped and disposed in a conductive casing contacting a connectingtenninal located on the periphery of said unit, whereby charges producedunder the effect of forces acting on said crystals may be conducted tosaid terminal to provide an electrical indication of said forces.

3. A piezomeasuring device according'to claim 2, further including anouter cylindrical jacket surrounding the outer portion of said platesand an inner cylindrical jacket disposed adjacent the inner surface ofsaid plates.

4. A piezomeasuring device according to claim 3, further including apair of insulating rings disposed between said casing and said outer andinner jackets, respectively.

5. A piezomeasuring device according to claim 4, wherein said piezodiscsare held between said pair of force-transmitting plates by a ring-shapedelectrode and a ring-shaped insulator disposed between said plates.

6. A piezomeasuring device according to claim 5, wherein one unit ofsaid plurality of transducer units is sensitive only to axialcompressional forces, while another unit is sensitive only to shearforces in a first direction.

7. A piezomeasuring device according to claim 6, wherein an additionaltransducer unit in said plurality of transducer units is sensitive onlyto rotary moments.

8. A piezomeasuring device according to claim 7, wherein anothertransducer unit in said plurality of transducer units is sensitive onlyto shear forces in a second direction different from said firstdirection.

9. A piezomeasuring device according to claim 8, wherein said forcereceiving body is a ring of U-shaped cross section in which eachtransducer unit is assembled with axial prestress, so that saidforce-transmitting members are resilient relative to said forcereceiving body.

10. A piezomeasuring device according to claim I, wherein the individualpiezocrystals are of approximately circular disclike form and havesubstantially the same dimensions independently of the crystal cut, andare inserted into retaining ring means operable to hold the sensitivityaxes of the individual crystals in a fixed position until assembly.

1 piezomeasuring device according to claim 10, wherein theforce-transmitting members are constructed as discs with a centralopening and are connected to each other under stress by thin-walledtubular members.

12. A piezomeasuring device according to claim I, wherein saidforce-transmitting members are constructed as discs with a centralopening and are connected to each other under stress by thin-walledtubular members.

13. A piezomeasuring device according to claim I, wherein the forcereceiving body is a ring of U-shaped cross section in which themeasuring units are assembled with axial prestress in such manner that aforce-transmitting member is resilient relative to the force receivingbody.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,614,488 Dated October 19, 1971 lnventofls) HANS CONRAD SONDEREGGER ETAL It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Page 1, column 1, line 8,

"Eisner" should read --Kist1er- Signed and sealed this 9th day of May1972.

EDWARD PLFLETCHER ,JR.

Commissioner of Patents Attesting Officer RM PO-1050 (10- I USCOMM-DCGOSTG-F'Gi ILSI GOVERNMENT PRINTING OFFICE I969 0-355-384

1. A piezomeasuring device with a force-receiving body for measuring aplurality of forces impinging thereon having a plurality of separatetransducer units mounted together, each transducer unit comprising: apair of force-transmitting members disposed about a common axis; and aplurality of piezocrystals disposed between the forcetransmittingmembers of said pair and being sensitive to forces in a single directionwith respect to said axis; wherein the crystals of each separatetransducer unit are sensitive to forces in a direction different fromthe direction of sensitivity of the crystals of the other transducerunits, whereby said measuring device provides a compact arrangementsensitive to forces in a plurality of directions.
 2. A piezomeasuringdevice according to claim 1, wherein said Pair of force-transmittingmembers of each transducer unit are annular plates, between which saidcrystals are arranged, said crystals being disc-shaped and disposed in aconductive casing contacting a connecting terminal located on theperiphery of said unit, whereby charges produced under the effect offorces acting on said crystals may be conducted to said terminal toprovide an electrical indication of said forces.
 3. A piezomeasuringdevice according to claim 2, further including an outer cylindricaljacket surrounding the outer portion of said plates and an innercylindrical jacket disposed adjacent the inner surface of said plates.4. A piezomeasuring device according to claim 3, further including apair of insulating rings disposed between said casing and said outer andinner jackets, respectively.
 5. A piezomeasuring device according toclaim 4, wherein said piezodiscs are held between said pair offorce-transmitting plates by a ring-shaped electrode and a ring-shapedinsulator disposed between said plates.
 6. A piezomeasuring deviceaccording to claim 5, wherein one unit of said plurality of transducerunits is sensitive only to axial compressional forces, while anotherunit is sensitive only to shear forces in a first direction.
 7. Apiezomeasuring device according to claim 6, wherein an additionaltransducer unit in said plurality of transducer units is sensitive onlyto rotary moments.
 8. A piezomeasuring device according to claim 7,wherein another transducer unit in said plurality of transducer units issensitive only to shear forces in a second direction different from saidfirst direction.
 9. A piezomeasuring device according to claim 8,wherein said force receiving body is a ring of U-shaped cross section inwhich each transducer unit is assembled with axial prestress, so thatsaid force-transmitting members are resilient relative to said forcereceiving body.
 10. A piezomeasuring device according to claim 1,wherein the individual piezocrystals are of approximately circulardisclike form and have substantially the same dimensions independentlyof the crystal cut, and are inserted into retaining ring means operableto hold the sensitivity axes of the individual crystals in a fixedposition until assembly.
 11. A piezomeasuring device according to claim10, wherein the force-transmitting members are constructed as discs witha central opening and are connected to each other under stress bythin-walled tubular members.
 12. A piezomeasuring device according toclaim 1, wherein said force-transmitting members are constructed asdiscs with a central opening and are connected to each other understress by thin-walled tubular members.
 13. A piezomeasuring deviceaccording to claim 1, wherein the force receiving body is a ring ofU-shaped cross section in which the measuring units are assembled withaxial prestress in such manner that a force-transmitting member isresilient relative to the force receiving body.